[Gllug] OT: MTU sizes, w/ Broadband

Chris Bell chrisbell at overview.demon.co.uk
Wed May 14 11:00:54 UTC 2003

On Wed 14 May, Murray wrote:
> so I've been reading about how BT are suggesting MTU size is changed to
> 1458, rather than (the common default?) of 1500.  (this isn't new, I think
> it surfaced dec 2002)
> http://www.adslguide.org.uk/newsarchive.asp?item=929
> It also refrences another article saying BT are going to fix it at their
> end.  Either way, I'm less concerned here about whether BT is giving good
> advice or not, and more about the concept of MTU & it's implementation.

   The problem is that BT have been using their ATM network to carry the
data. Their network was specifically designed for speech using two parallel
data streams, one carrying route data and the other digitally coded
real-time audio, using very small packet sizes. It was probably very
advanced at the time it was designed, perhaps it was a work around when many
large but slow and simple chips were required, although few people
understand the reasons now. The optimum MTU for this system works out at
1458, until they can install new equipment designed to work with an MTU of
1500, although the affect is minimal unless their system approaches

Chris Bell

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